Sunday, 23 September 2012

Fossil Porpoises Muddy the Waters

Harbour porpoise

Dolphins and porpoises belong to two separate families, and there are clear differences between them. However, this isn't immediately obvious at first glance: they're both small cetaceans, with flippers, tail flukes, and so on. A comparison between, say, a bottlenose dolphin (Tursiops truncatus) and a harbour porpoise (Phocoena phocoena) does show a number of differences that are broadly true of their respective relatives - but not entirely so.

Porpoises, for instance, are smaller than dolphins, on average. But only 'on average', because, in fact, the smallest dolphin is smaller than the smallest porpoise. Similarly, dolphins usually have a 'beak' - a narrow snout projecting forward from the front of the head - that is missing in the more rounded heads of porpoises. But, once again, there are dolphins that don't have the beak, such as Risso's dolphin (Grampus griseus).
Nonetheless, dolphins and porpoises are clearly distinct, and for a number of reasons, relating to both their behaviour, and their anatomy. As is so often the case with mammals, one sure way to tell them apart (assuming you can get close enough) is by examining their teeth. Both groups possess an unusually large number of teeth which, unlike those of most land-dwelling mammals, are essentially the same throughout the jaw. That is, there are no chewing molars, stabbing canines, and the like, just lots of piercing teeth that evolved to grasp slippery fish and swallow them more or less whole. But the teeth of dolphins are pointed and conical, while those of porpoises are flattened from side to side, like a spatula.

Dolphins, like other toothed whales, also have a weirdly asymmetrical skull, which probably has something to do with their echolocation abilities. Although the porpoise skull isn't truly symmetrical, as it would be in a land-dwelling mammal (or seals, for that matter), it is much more so than that of dolphins - even though, they, too, have sonar. There are other clear differences as well, especially, for example, in the bones around the blow-hole. Another oddity is that the skeletons of adult porpoises look curiously immature, as if they were really infants that haven't yet finished growing. Nobody is really sure why this should be.

Today, there are over three dozen species of dolphin, found across all of the world's oceans, often far from land. However, there are only six living species of porpoise, generally found close to the shore, and not so widespread as dolphins. Even so, there are porpoises in the eastern Indian Ocean, in the North Atlantic, around southern South America, and across much of the North Pacific. All but one of the known fossil species, however, come from the Pacific, and it is generally accepted that this where the porpoise family originated, spreading out to other oceans some time during the Pliocene.

But can we be more precise than that, and can fossil evidence help us to decide how dolphins and porpoises relate to one another? The announcement of three new fossil species in two separate papers published this month provides some new information... and only shows that the story is more complicated than previously thought.

The generally accepted story is that porpoises probably evolved off the coast of North America, because this is where most fossils have so far been found. Indeed, the oldest known porpoise fossil, Salumiphocoena, comes from the coast of California, and dates to somewhere between 9 and 12 million years ago, during the late Miocene epoch. The three new fossils, described by Mizuki Murakami and co-workers, are all from Hokkaido, in northern Japan, and therefore tell us something about what was going on on the opposite side of the Pacific.

Two of the fossils, named as Archaeophocoena and Miophocoena, date from around 6 million years ago, not long before porpoises are thought to have spread beyond the Pacific, and showing that they were already well established off the coast of Asia by this time. The other, Pterophocoena, is much older, dating back just over 9 million years, and therefore just possibly living at the same time as the very oldest porpoise we previously knew of. Certainly, it indicates that porpoises travelled west earlier than we'd realised.

The authors conducted an analysis of their fossils, comparing various features of their skulls with those of a range of other porpoises, dolphins, and whales. By considering the number and nature of the features they have in common, and analysing them on a computer, its possible to draw up a family tree showing how they might be related. It's here that the complications come in.

Let's look at the older fossil first. Some crucial parts of the skull, especially around the blow-hole, are missing, but from what we do have its possible to confirm that it certainly is a porpoise: it has a number of features unique to porpoises, and, so far as we can tell, none that we'd expect to find only in dolphins. It even has the strange immaturity of the skeleton that modern porpoises have. It's possible, of course, that the specimen in question genuinely was immature when it died, but since it's slightly larger than any living species, that seems unlikely.

On the other hand, though, it's also the case that many features we'd expect to find in porpoises aren't there.

In some respects, that's not so odd. In fact, there's not one single fossil porpoise we know of that has had the full set of features you'd use to distinguish the skeleton of a living porpoise from a dolphin. For one thing, unlike the living forms, every fossil ever discovered has had a noticeably asymmetrical skull. Living porpoises, it seems, descended from rather more dolphin-like ancestors and only later changed to their modern form. Somewhere in their evolutionary past, their ancestors lost the symmetry of their skulls, and then, when the modern forms appeared, they gained it back again. We don't know why.

What is interesting, though, is that, according to the analysis, Pterophocoena has even less features in common with modern porpoises than we'd expect. Less than any other fossil, in fact, including the one that's apparently older. In technical parlance, it is the most 'basal' porpoise yet known.

That doesn't necessarily mean it's the most 'primitive'. What it shows is that, at some unknown point in the past, a population of early porpoises, probably off the coast of North America, split in two, with one group heading off to Asia. Most of the other features we associate with later porpoises evolved in the group that stayed behind, but that doesn't mean that the ancestors of Pterophocoena didn't themselves evolve as time went by. They just did so down a different path. Perhaps they lost some of the features that Salumiphocoena and its relatives retained, or perhaps those appeared in the North American line later; there's no way to tell which.

Turning to the other two species, they have enough in common to suggest that they are close relatives of one another. However, they too, have relatively few features shared with other porpoises, although not so little as Pterophocoena, and so clearly belonging to the same family. They're unlikely to be descended from the earlier species, and so presumably represent a second wave of arrivals from the east that replaced the older forms. Since there are younger Japanese fossil porpoises that more closely resemble living species, the same story has apparently happened at least twice since. (Assuming, that is, that North America really is the origin of the group - its always possible there could have been some migration going in the opposite direction, instead).

Unlike the older fossil, these ones both include the top of the skull, including the crucial blow-hole. And here things get stranger, because these parts of the fossils really look quite dolphin-like. Given that older fossil porpoises tend to look quite dolphin-like anyway, that wouldn't be so odd if the oldest fossil porpoise - and it's quite a lot older, remember - had the same feature, but it doesn't. What is going on here? Perhaps its a case of parallel evolution, a previously lost feature re-appearing again in one line, but not another, or perhaps the evolutionary story is even more tangled than it appears already.

Either way, all of these fossils seem to suggest a fairly close relationship between porpoises and dolphins, perhaps dating back 12 to 15 million years. Comparing them more broadly with a range of other whales, that was the pattern the researchers found, and the one shown in the diagram above. You might not think that's much of a surprise, given how similar they look, but actually it is.

That's because we have some pretty good evidence, especially from molecular genetics, that dolphins aren't the closest living relatives of porpoises at all. Instead, those are the 'white whales' - narwhals and belugas. The authors of these papers, describing the fossil porpoises, believe that their analysis is more accurate than the previous studies, and that the association with white whales is probably wrong. I'm not so convinced, because the existing evidence does look fairly strong. But it isn't infallible, and there are some valid possibilities that could explain it giving a misleading picture.

Thing is, one could probably say the same about the fossil evidence. I don't know which is better, but I think it's worth noting that the fossil history of white whales is quite poor. There are only two living species, both of them quite specialised, which makes it hard to say quite how they're related to anything that doesn't look as strange as they do. And there are only two fossil species - one of them itself only described earlier this year - which makes it difficult to put together much of a picture there, either.

Honestly, we don't know. Are porpoises more closely related to dolphins or to narwhals? The new fossils raise at least as many questions as they answer, and there is clearly much more science to be done. That there always seem to be more questions is part of what makes science so endlessly fascinating.[Picture by Erik Christensen, from Wikimedia Commons. Cladogram adapted from Murakami et al 2012.]